In recent years it has become more and more difficult for the author or copyright owner of a passage of text to control what happens to that text once it has been released into the public domain. If the text is released in electronic format, it can be repeatedly copied and distributed across the Internet in seconds. Even if it is released only in hardcopy, it is a trivial matter for a third party to copy or scan the relevant page and then to use optical recognition (OCR) technology to generate an electronic version.
There are a variety of circumstances where the need arises to detect when a particular passage of text has been copied. First, if the text passage is copyright protected, the copyright owner may legitimately wish to know who is making unlawful copies of the passage; alternatively, the copyright owner may be interested to know how many times an authorised licensee has made a copy, so that license royalties may be calculated. Second, there is a need to detect plagiarism, especially amongst students who may copy passages of text from the Internet, and pass them off their own work. Thirdly, there is the security field in which, for reasons of national security or otherwise, government agencies may wish to restrict or prohibit the copying of certain secret documents.
In all of these scenarios, it would be desirable to detect not only electronic copying, but also the copying of physical printed documents, for example by scanning or photocopying.